Inter-rebel alliances may increase the odds of rebel survival or victory through resource-sharing and coordination of activities. Given the benefits, why are some groups more successful in alliance making than others? Drawing on hegemonic stability theory, I argue that the presence of foreign sponsor is likely to encourage alliance formation in civil wars. If two rebel outfits share a common state sponsor, they are more likely to enter an alliance, than if only one of them receives foreign support. Shared sponsors may demand cooperation between their clients and credibly threaten to punish them for non-compliance. They may also insist on the establishment of umbrella institutions to improve their monitoring and sanctioning capacity, and to increase the legitimacy of their clients. I test this argument using the UCDP Actor dataset with new data on alliances between rebel groups. I find strong evidence that shared sponsors increase the probability of inter-rebel alliance.